Patent GB-1086907-A (GB)
Title: Improvements in or relating to methods and apparatus for measuring vapour pressure in vacuum system
Inventor: STECKELMACHER WALTER
IPC: G01L9/00
Filed: 1965-06-25
Abstract: 1,086,907. Electric vapour pressure transducers. EDWARDS HIGH VACUUM INTERNATIONAL Ltd. June 20, 1966 [June 25, 1965], No.27093/65. Heading G1N. The vapour pressure in vacuum systems is measured by detecting changes in frequency or in the Q factor of a Piezo-electric vibrator, whose crystal is coated with sorptive material, the changes in frequency resulting from changes in the mass of the crystal due to sorption of the vapour. A similar arrangement to the above can be used for measuring vapour flow rate in, e.g. the pumping line of a vacuum system, by placing two vapour pressure measuring systems at two points along the vapour flow line, the difference in vapour pressures at the two points being representative of the vapour flow rate. In the embodiment for measuring vapour pressure, Fig.1, a quartz crystal 4, forming part of an oscillator 5, e.g. a Colpitts oscillator, and coated on one or both sides with a thin sorptive layer of magnesium fluoride or aluminium oxide, is placed in the pumping line 2, by which a processing chamber 1 is evacuated by pumping equipment 3. The output from the oscillator is fed to a diode mixer circuit 6, to which the output from a reference oscillator 7, identical to the oscillator 5 and having a crystal 8 similarly coated with non-sorptive layers on one or both faces to give the same fundamental frequency as that of the crystal 4 before sorption of vapour, is also fed. The output of the mixer 6 is then fed to a pulse shaping circuit 9 and a counter 10, whose D.C. output signal is representative of the frequency difference between the oscillator 5 and 7 and hence the vapour pressure at the crystal 4. This signal is registered on a display meter 11. In an alternative arrangement, in which the Q factor of the oscillator is measured, two similar crystals are placed in the region whose vapour pressure is to be measured, one having a sorptive coating and the other being untreated, each crystal being associated with an oscillator and a Q-factor determining circuit. The output from each of these latter circuits is fed into a difference circuit, whose output is representative of the amount of vapour absorbed by the coated crystal and therefore of the vapour pressure. In one arrangement for measuring vapour flow, Fig.2 (not shown) two crystals (4, 4a), spaced apart down the length of a vapour flow line (2), are provided, each associated with an oscillator (5, 5a), whose outputs are compared in mixers (6, 6a) with the output from a reference oscillator (7). The D.C. outputs from counters (10, 10a) are then compared in a difference circuit (13), whose output is representative of the difference between the vapour pressures at the points (4, 4a). This signal may be displayed on a display meter, suitably calibrated to give a reading of vapour flow along the line 2. The signal from the difference circuit may also be fed into a process controller (14), connected to the pumping equipment (3), which may serve to terminate a process, e.g. a drying process, or to control the rate of vapour flow. In an alternative modification, the arrangement of Fig.1 could be adapted directly to measure vapour flow, the reference oscillator crystal 8 being placed in the vapour flow line in place of the crystal (4a) in Fig.2, no further reference oscillator being utilized. The vapour flow rate can also be measured by comparing the outputs from two vapour pressure measuring circuits in which the difference between the Q factors of two oscillators one with a coated crystal and one with an uncoated crystal is determined. The resulting difference signal giving an indication of the vapour flow rate along the line, this signal being fed into a suitable display meter or utilized, as described with reference to Fig. 2, for controlling the pumping equipment. In all the embodiments, the Piezo-electric crystal used is an AT cut quartz crystal, secured in its holder by spring clip type contact electrodes, e.g. of silver or gold, whose contact areas are smeared with silver impregnated paste to ensure reliable contact. The sorptive layers of magnesium fluoride or aluminium oxide are deposited on one or both faces of the crystal by vacuum deposition.